1. Field of the Invention
This invention relates to cold rolled steel sheets to be enamel coated having improved press moldability and weldability as well as improved enameling properties, and a method for making the same.
2. Prior Art
Steel sheets to be coated with porcelain enamel or ceramic have to exhibit excellent enameling properties including enamel adherence, baking distortion, and fish-scaling resistance. In addition, they are required to be press moldable because most enameled steel stocks are press molded into end products under relatively severe conditions.
It is well known in the art, for example, from Japanese Patent Publication Nos. 42-12348 and 44-18066 that titanium steels, particularly those steels based on super-low carbon steel with not more than 0.02 wt % C having titanium added thereto have high press moldability. Techniques for improving enameling properties while taking advantage of the press moldability of titanium steel are disclosed in Japanese Patent Publication No. 45-40655 and Japanese Patent Application Kokai Nos. 53-131919 and 56-9357.
These techniques are effective in improving scaling resistance among other enameling properties while preserving good press moldability. By utilizing the tendency of titanium to form carbide, sulfide, and nitride in steel, these techniques increase the hydrogen occlusion capacity of steel in order to minimize the scaling susceptibility.
Titanium must be added in a sufficient amount to exert its effect to a full extent. Large amounts of titanium added, however, adversely affect enamel adherence. For this reason, titanium steel could find only a limited range of utility as steel stock to be enamel coated although it possessed excellent press moldability and scaling resistance.
After steel sheets to be enamel coated are worked into shapes, they are often welded by a variety of methods. As compared with capped steel commonly used as enameled steel, titanium steel, however, is inefficient in welding operation and there often appear deficiencies such as blowholes in welded zones. Particularly, significant "shrinkage" occurs in welded zones, which develops itself in stripe form after enamel baking. Blowholes lead to bubble defects. Capped steel is thus primarily used in such applications requiring welding where titanium steel has not found utility.